In recent years, research and development have been extensively conducted on light-emitting elements which utilize electroluminescence. In a basic structure of such a light-emitting element, a substance having a light-emitting property is interposed between a pair of electrodes. By application of voltage to the element, light emission can be obtained from the substance having a light-emitting property.
Since such a light-emitting element is a self light-emitting type, there are advantages that visibility of a pixel is better than visibility of a liquid crystal display, that a backlight is not necessary, and the like. Accordingly, such a light-emitting element is suitable for a flat panel display element. In addition, other advantages of such a light-emitting element are that the element can be manufactured to be thin and lightweight and the response speed is very high.
Since the light-emitting element can be formed into a film shape, surface light emission can be easily obtained by formation of a large-area element. This is a feature which is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element has a high utility value as a surface light source applicable to a lighting system and the like.
Light-emitting elements which utilize electroluminescence are classified broadly according to whether they use an organic compound or an inorganic compound as a light-emitting substance.
When an organic compound is used as a light-emitting substance, electrons and holes are injected into a layer containing a light-emitting organic compound from a pair of electrodes by voltage application to a light-emitting element, so that current flows therethrough. The electrons and holes (i.e., carriers) are recombined; thus, the light-emitting organic compound becomes in an excited state. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Based on such a mechanism, such a light-emitting element is referred to as a current-excitation type light-emitting element.
As types of excited states of the organic compound, there are a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio thereof in the light-emitting element is S*:T*=1:3.
In a compound which converts a singlet excited state into light emission (hereinafter referred to as a fluorescent compound), light emission from a triplet excited state (phosphorescence) is not observed at a room temperature but only light emission from a singlet excited state (fluorescence) is observed. Therefore, in a light-emitting element with the use of a fluorescent compound, the theoretical limit of internal quantum efficiency (the ratio of generated photons to injected carriers) is considered to be 25% based on S*:T*=1:3.
On the other hand, when a compound which converts a triplet excited state into light emission (hereinafter referred to as a phosphorescent compound) is used, internal quantum efficiency can be theoretically improved from 75 to 100%. That is, luminous efficiency can be three to four times as high as that of a fluorescent compound. From such a reason, in order to achieve a high efficiency light-emitting element, a light-emitting element with the use of a phosphorescent compound has been actively developed recently.
When a light-emitting layer of a light-emitting element is formed with the use of the above phosphorescent compound, in order to suppress concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation (T-T annihilation), the light-emitting layer is often formed so that the phosphorescent compound is dispersed in a matrix of another substance. At this time, the substance which serves as a matrix is referred to as a host material, and the substance which is dispersed in a matrix such as a phosphorescent substance is referred to as a guest material.
When the phosphorescent compound is used as a guest material, a host material is needed to have higher triplet excitation energy (an energy difference between a ground state and a triplet excited state) than that of the phosphorescent compound. In Patent Document 1 (Japanese Published Patent Application No. 2002-352957), TAZ is used as a host material of a phosphorescent compound, which emits green light.